Polarizers included in polarizing plates are optical devices for polarizing natural light or arbitrarily polarized light in a particular direction, and are widely used in display devices such as liquid crystal displays (LCDs) or organic light emitting diode (OLED) displays. Currently, polyvinyl alcohol-based polarizing films, containing an iodine-based compound or a dichroic dye and having a molecular structure in which molecular chains are oriented in a particular direction, are generally used as polarizers of display devices.
Polyvinyl alcohol-based polarizing films are manufactured by dyeing a polyvinyl alcohol-based film with iodine or a dichroic dye, stretching the polyvinyl alcohol-based film in a particular direction, and performing a cross-linking process thereon. In this case, the stretching process may be a wet stretching process using a solution such as an aqueous boric acid solution or an aqueous iodine solution or a dry stretching process performed in the atmosphere, and the stretching ratio of the stretching process is generally 5 times or greater. In such a manufacturing process, the thickness of a polyvinyl alcohol-based film may be 60 μm or greater before stretching so as to prevent breakage during stretching. If the thickness of the polyvinyl alcohol-based film is less than 60 μm before stretching, the degree of swelling of the polyvinyl alcohol-based film may increase, and the polyvinyl alcohol-based film may easily be broken during stretching because the modulus of electricity of the polyvinyl alcohol-based film per unit area is high due to a thin thickness.
Recently, polarizing plates have been required to have reduced thicknesses to allow for the use thereof with thinner display devices. However, if polyvinyl alcohol-based films having a thickness greater than 60 μm are used as described above, there is a limit to reducing the thickness of polarizers. Thus, much research has recently been conducted into developing thinner polarizers. For example, according to a recently proposed method for manufacturing a thin polarizer, a resin layer is formed on a base film by coating the base film with a polyvinyl alcohol-based resin and drying the polyvinyl alcohol-based resin, and then a stretching process is performed. In the method, a crystalline ester-based resin or an amorphous ester-based resin is used as the base film. Specifically, a crystalline ester-based resin such as a crystalline polyethylene terephthalate resin, or an amorphous ester-based resin such as an amorphous polyethylene terephthalate resin is used as the base film.
However, if a crystalline polyethylene terephthalate resin is used, the ratio of stretching is limited to 5 times, and if an amorphous polyethylene terephthalate resin is used, the ratio of stretching is limited to 5.5 times. Therefore, a high-ratio stretching process may not be performed, and thus there is a limit to improving optical characteristics. In addition, if a polyethylene terephthalate resin is used as described above, since the degree of crystallization of the polyethylene terephthalate resin is high and a high tensile force is required because of the structure and characteristics of the polyethylene terephthalate resin and the conditions of a stretching process performed after a melting-solution casting process, a dry stretching process may be performed only at a high temperature of about 120° C. to about 170° C. Therefore, to perform a dry stretching process at a high temperature of about 120° C. to about 170° C., an additional heating process may be required, or process costs may be increased. Furthermore, since a polyvinyl alcohol film formed on a base film has a glass transition temperature of about 70° C. to about 80° C., the polyvinyl alcohol film may be damaged or properties thereof may be changed during a high-temperature dry stretching process. In addition, if a polyethylene terephthalate resin is used as described above, since a base film formed of the polyethylene terephthalate resin is difficult to stretch, cross-linking of polyvinyl alcohol by a boric acid may be increased during a wet stretching process using an aqueous boric acid solution, and in this case, the ratio of stretching may be decreased. In addition, if a polyethylene terephthalate resin is used as described above, since a base film formed of the polyethylene terephthalate resin is difficult to stretch, cross-linking of polyvinyl alcohol by a boric acid may be increased during a wet stretching process using an aqueous boric acid solution, and in this case, the ratio of stretching may be reduced.
In addition, if the above-described coating method or a co-extrusion method is used, it may be difficult to separate a polyvinyl alcohol layer and a base layer from each other after stretching, and since a large amount of peel force is used for separating the layers, the polyvinyl alcohol layer may easily be damaged or deformed during separation. As a result, optical characteristics of a polyvinyl alcohol-based film such as a degree of polarization may deteriorate. Furthermore, if a coating method or a co-extrusion method is used, since a polyvinyl alcohol resin is melted and extruded or is made into a coating solution and applied, the physical properties of a polyvinyl alcohol film may be easily varied according to extrusion conditions, coating conditions, or film-forming conditions. Therefore, the physical properties of final polyvinyl alcohol products may deteriorate, and it may be difficult to obtain final polyvinyl alcohol products having uniform physical properties.